连续能量核数据敏感性分析及其在AP1000中的应用

AP1000是典型的第三代核电技术，对AP1000反应堆进行核数据的敏感性分析是不确定度量化分析的基础，对AP1000后续的安全分析有重要作用。本文基于反复裂变几率方法在蒙特卡罗前向计算中求解共轭通量，并根据一阶微扰理论得到keff对核数据的灵敏度系数。针对反复裂变几率方法普遍存在占用内存大的问题，采用稀疏矩阵的存储方式降低内存。针对计数效率低、统计涨落大的问题，采用重叠块法提高计数效率。通过在蒙特卡罗程序NECP-MCX中开发连续能量核数据敏感性分析功能模块，并对AP1000进行连续能量核数据灵敏度系数的计算，得到了对keff的灵敏度系数影响较大的核数据，同时将计算结果与MCNP6进行了比较。结果表明，NECP-MCX和MCNP6的计算结果吻合较好。

Sensitivity Analysis of Continuous-energy Nuclear Data and Its Application in AP1000

AP1000 is the typical third-generation nuclear power technology. The sensitivity analysis of continuous-energy nuclear data for AP1000 is the basis of uncertainty quantification which plays an important role in subsequent safety analysis for AP1000. Multi-group approximations exist in sensitivity analysis based on multi-group energy nuclear data. Therefore, it’s necessary to calculate sensitivity coefficients based on continuous-energy nuclear data. In this paper, adjoint flux was calculated based on iterated fission probability (IFP) in Monte Carlo forward calculation and then sensitivity coefficients of keff to nuclear data were obtained through first-order perturbation theory. The sparse-matrix method was used to reduce the large memory usage in IFP, while the overlapping block method was used to improve counting efficiency and large statistics fluctuation. The function module of sensitivity analysis was developed in Monte Carlo code NECP-MCX. The module developed above was verified based on the VERA 2B problem. Both energy-dependent and energy-independent sensitivity coefficients calculated by NECP-MCX are all within 3σ of those obtained by MCNP, which indicates the accuracy of the NECP-MCX. In addition, the sensitivity coefficients related to the scattering cross section fluctuate greatly, which is because when the magnitudes of the scattering term and the total term in the sensitivity coefficient formula are the same, the subtraction of these two terms will result in a value with a larger statistical fluctuation. Furthermore, the sensitivity analysis of continuous-energy nuclear data was performed with enhanced NECP-MCX for AP1000 and the nuclear data that are most sensitive to keff were analyzed, which are 235U-ν, 235U-σf, 235U-σt, 238U-σγ, 1H-σelas and 10B-σt, where 235U-ν, 235U-σf and 235U-σt are mainly related to the production of neutron, 238U-σγ and 10B-σt are dominant in the disappearance of neutron and 1H-σelas is dominant in the slowing down of neutron. In this paper, the sensitivity coefficient of keff to nuclear data was derived based on first-order perturbation theory. The module of continuous-energy nuclear data sensitivity analysis was implemented in Monte Carlo code NECP-MCX. The verification of the module was conducted based on VERA 2B problem. Finally, the sensitivity coefficients of keff to continuous-energy nuclear data were calculated for AP1000. The most sensitive nuclear data to keff were obtained from the results. In the future, based on the obtained sensitivity coefficients and the covariance information in the nuclear data, the uncertainty of the AP1000 core keff calculation caused by nuclear data can be obtained through sandwich rule for subsequent safety analysis.